Power generation systems that provide low cost energy with minimum environmental impact, and that can be readily integrated into the existing power grids or rapidly sited as stand-alone units, can help solve critical power needs in many areas. Combustion engines such as microturbines or reciprocating engines can generate electricity at low cost with efficiencies of 25% to 40% using commonly available fuels such as gasoline, natural gas and diesel fuel. However, atmospheric emissions such as nitrogen oxides (NOx) and particulates can be a problem with reciprocating engines.
One method to generate electricity from the waste heat of a combustion engine without increasing the output of emissions is to apply a bottoming cycle. Bottoming cycles use waste heat from such an engine and convert that thermal energy into electricity. Rankine cycles are often applied as the bottoming cycle for combustion engines. A fundamental organic Rankine cycle consists of a turbogenerator, a preheater/boiler, a condenser, and a liquid pump. Such a cycle can accept waste heat at temperatures somewhat above the boiling point of the organic working fluid chosen, and typically rejects heat to the ambient air or water at a temperature somewhat below the boiling point of the organic working fluid chosen. The choice of working fluid determines the temperature range/thermal efficiency characteristics of the cycle.
Simple ORC Systems using one fluid are efficient and cost effective when transferring low temperature waste heat sources into electrical power, using hardware and working fluids similar to those used in the air conditioning/refrigeration industry. Examples are ORC systems using radial turbines derived from existing centrifugal compressors and working fluids such as refrigerant R245fa.
For higher temperature waste heat streams, the most cost-effective ORC systems still operate at relatively low working fluid temperatures, allowing the continued use of HVAC derived equipment and common refrigerant. However these systems, although very cost-effective, do not take full advantage of the thermodynamic potential of the waste heat stream.